Tissue proteins forms the basis for organ structure and function. Excessive losses of tissue protein can compromise organ function and eventually will result in death. Any stressful situation such as trauma and chronic debilitating diseases results in tissue losses that if sustained can compromise organ function. In most cases nutrition alone cannot prevent this tissue loss because of excessive breakdown of tissue proteins. Thus alternatives to nutrition must be used to abate or slow the protein wasting or excessive loss of body nitrogen.
Nitrogen balance is the difference between the nitrogen intake (as protein or amino acids) in an individual and the total nitrogen excretion. When the nitrogen intake equals the nitrogen excretion, the subject is in nitrogen equilibrium. If the nitrogen intake exceeds the nitrogen excretion, the nitrogen balance is positive, but if the nitrogen excretion is greater than the nitrogen intake, the nitrogen balance is negative. Nitrogen balance can be estimated by monitoring urinary nitrogen. Absolute nitrogen balance also requires fecal nitrogen measurement, but in most cases this does not change appreciably unless the diet is substantially altered. Thus, the nitrogen content of urine can be approximately correlated with total nitrogen excretion. Monitoring nitrogen content of urine is especially important where the patient has or is expected to have a persistent negative nitrogen balance.
Promoting nitrogen retention has therapeutic importance where the patient has been subjected to trauma or stress conditions which can be expected to induce potential loss. Injury (surgical, traumatic, and burn) and sepsis result in accelerated protein breakdown, which is manifested by increased nitrogen loss. Catabolic conditions are also frequently associated with severe bodily diseases such as cancer, AIDS, etc. Loss of muscle protein may occur due to normal aging, and consequently, protein sparing therapy may be indicated for elderly patients who are otherwise normal.
Therapeutic agents and certain nutritional regimes are known which can promote nitrogen retention. However, therapeutic options to decrease body nitrogen losses (protein wasting) are limited. Injections of certain hormones may improve nitrogen retention. Growth hormone injections can in the short term at least decrease tissue protein losses: Horber et al., J. Clin. Invest. (1990) 86: 256. Steroids such as testosterone when injected can decrease nitrogen loss: Daham et al., Metabolism (1989) 38: 197. These compounds have to be injected and may have undesirable side effects, limiting usefulness in disease states.
A nutritional approach to protein sparing was investigated by Dr. MacKenzie Walser and associates. They experimented with keto analogs of essential amino acids as partial or complete substitutes for the corresponding amino acids, for example, as supplementation to protein-reduced diets in uremia. [See, for example, Walser et al., J. Clin. Inv. (1973) 52: 678-690.] Experiments by Walser and associates demonstrated a nitrogen sparing effect from mixtures of branched-chain keto acids: Saiper and Walser, Metabolism (1977) 26: 301-308. Patents have issued to Walser on the use of keto analogs of essential amino acids for promoting synthesis and suppression of urea formation in humans (U.S. Pat. Nos. 4,100,161 and 4,101,293).
The keto acid analog of L-leucine is alpha-ketoisocaproate (KIC) which is also sometime referred to as "keto-leucine". KIC does not have L and D forms as does leucine. It is known that there is an interconversion of circulating KIC and leucine. Published studies have demonstrated that KIC can be substituted in animal diets for leucine providing that larger molar amounts of KIC are used. Chawla et al. reported that weight loss by rats being fed a diet deficient in leucine could be prevented by adding KIC to the diet, but the efficiency of substitution was only 20 to 27%. [J. Nutr. (1975) 105: 798-803], and Boebel et al. reported that the efficiency of KIC was only about 56% with reference to leucine [Boebel and Baker, J. Nutr. (1982) 112: 1929-1939].
Dr. Steven L. Nissen of Iowa State University, Ames, Iowa, U.S.A. has carried out studies with domestic animals in which KIC is incorporated in the animal feeds. As described in his U.S. Pat. No.4,760,090, it was found that ketoisocaproate (KIC) when fed to cattle and sheep can result in enhancement of growth and feed efficiency. In another use of KIC feeding, egg production of laying chickens was increased (U.S. Pat. No.4,764,531). In later experiments carried by Dr. Nissen at Iowa State University, .beta.-hydroxy-.beta.-methylbutyric acid (HMB) was fed to domestic animals. The effects obtained with HMB feeding were different than with KIC.
Metabolically, KIC and HMB are not directly related. KIC is the only metabolic product of leucine, while HMB is a minor product in the metabolic sequence of KIC. Leucine is either used for protein synthesis in the body or is converted directly to KIC. In the mitochondria, KIC is decarboxylated to isovalarylCoA and then further metabolized to ketone bodies. In certain disease conditions, such as isovalaric acidemia, an alternate oxidative pathway for KIC has been observed, which appears to produce .beta.-hydroxy-.beta.-methyl-butyrate (HMB). In atypical cases, such as a genetic absence of the dehydrogenase enzyme, there is evidence that HMB can accumulate in the urine: Tanaka, et al. Biochim. Biosphys. Acta. 152: 638-641 (1968). Also, in acidosis conditions, HMB levels can be increased in urine: Landass, Clin. Chim. Acta. 64: 143-154 (1975).
The differing activities of HMB as fed to domestic animals provided the basis for several patents by Dr. Nissen. U.S. Pat. No. 4,992,470 discloses administration of HMB for enhancing the immune response of mammals, and/or as an ingredient in the raising of meat producing animals (viz. ruminants and poultry) to increase lean tissue development. (See also U.S. Pat. Nos. 5,087,472 and 5,028,440.)